Ammonia denitration has been used to remove nitrogen oxides (hereinafter referred to as "NOx") from gas containing oxygen exceeding in quantity the oxidizing capacity of the reductive gas, particularly exhaust gas from combustion at a higher air/fuel ratio than stoichiometric value. However, this process cannot be applied to small combustors, for the following two reasons. First, it requires an ammonia source. Secondly, "slip" of ammonia in excessive quantity can cause a secondary environmental pollution problem. Recently it was found, as disclosed in Japanese Patent Laid Open Sho 63-100919, that NOx can be selectively reduced by hydrocarbons on a zeolite catalyst ion-exchanged with Cu or the like metal.
However, if hydrocarbons having four or fewer carbons are used as a reducing agent, the above-mentioned catalyst provides low selectivity (molar ratio of hydrocarbons used in NOx reduction to total hydrocarbons consumed) and therefore low NOx conversion, in the presence of the water vapor always contained in ordinary exhaust gas.
Armor, et al. reported (Applied Catalysis B: Environmental, vol. 1, p. L31) that NOx can be selectively reduced by methane on Co ion-exchanged ZSM-5 (MFI zeolite). It is known, however, that the catalytic activity is also decreased in the presence of water vapor, so that the catalyst does not have a sufficient activity for practical use. Therefore, a catalyst which is effective even in the presence of water vapor is awaited.
As a solution to the above-mentioned conventional problems, Italian Patent Application No. MI93A2337 discloses a NOx reduction process which uses a BEA zeolite ion-exchanged with cobalt (Co-BEA).
The Co-BEA offers substantial improvement in catalytic activity and durability at low temperature in actual exhaust gas conditions (in the presence of water vapor etc.). However, if the concentrations of NOx and reductant hydrocarbon are low, the reaction rate decreases on this catalyst, and a sufficient NOx conversion for practical use cannot be achieved. Therefore, more active catalyst has been desired